(A) says, "Although insecticides are effective in ridding the environment of insect pests, they often kill beneficial insects at the same time. Since these beneficial insects are so important, we must find other ways to combat insect pests."
The problem with (A) is that its principle sounds a lot more like costs outweighing benefits. However, there's nothing about short term vs. long term in this answer choice (as there was in the stimulus), so we can get rid of this answer choice. We need the correct answer to say something about short term vs. long term costs.
(C) says, "It is unwise to use highway maintenance funds for construction of new roads. There is some immediate benefit from new roads, but if these funds are not used for maintenance, the total maintenance cost will be greater in the long run."
(C) looks great. There is an immediate benefit that is outweighed by long term costs, so this answer choice is saying something about short term vs. long term costs. This matches the stimulus well, so it's the correct answer choice.
Hope this helps. Let us know if you have any other questions!